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Newswise — Andromeda is the nearest 'large' galaxy to the Milky Way, at 2.5 million light years away. Its size and disc-like structure is similar to the Milky Way, and it's often referred to as our companion galaxy. The next closest large galaxy is 10 million light years away.

This new study observes 13 smaller satellite galaxies orbiting around the immense Andromeda galaxy in a way similar to how the planets in our solar system orbit around the sun. The galaxies are orbiting on a thin, pancake-like plane at a scale 900 million times larger than our own solar system.

"These dwarf galaxies appear to have formed together in the structure, and our analysis is showing their stars seem to have formed at a similar epoch in the early universe," says Peter Smith, a Dalhousie University graduate student who works on follow up research.

The Lambda-Cold Dark Matter (Lambda-CDM) model struggles to explain the formation of these thirteen satellite galaxies orbiting around Andromeda. The Lambda-CDM is a standard model in astrophysics that assumes galaxies collide and merge with one another to grow mass.

"This tells us that this hierarchical buildup that gravitational simulations predict isn't quite right—as structures like this 'andromeda pancake' never happen in the simulations. We may not understand gravity as well as many would like to believe. It's very hard to test gravity in the very weak field limit," explains Dr. Scott Chapman, a co-author of the study and a professor at Dalhousie University.

The discovery also lends support to theories on how satellite galaxies are orbiting around the Milky Way, which is a much harder activity to observe given the earth's position within it—the galaxy itself blocks our view.

The researchers are working on a followup study that will explore how the orbital system formed the way that it did, working on the hypothesis that all thirteen galaxies must have formed at the same time within the same structure billions of years ago.